When pumping a production well, it is desirable to run the pump at a speed where the pump production closely matches the maximum production of the well. This is achieved by pumping the well down as close as possible to the pump inlet to minimize back pressure of liquid in the well annulus on the oil-bearing formation and, thus, maximize the flow of formation fluid to the pump. However, the liquid level may drop below the level of the pump inlet resulting in burnout of the pump. It is therefore necessary to ensure that the liquid level remains within a range which allows for optimum well production but prevents pump burnout due to pump-off.
A number of systems are known for this purpose which simultaneously measure the load on the rod string as well as its position. Load cells are used and are mounted on the rod string of the pump.
U.S. Pat. No. 3,951,209 discloses a method for calculating the energy output to the rod by integrating the product of the load on the rod and the displacement of the rod. If a reduction in energy input to the rod is detected, a signal is produced to trigger shut down of the pump.
U.S. Pat. No. 4,286,925 describes a system wherein the pump is shut down when the rod string load exceeds a preset value on the downstroke which signals pounding due to pump off or is smaller than a preselected value on the upstroke thereby signaling rod failure. Again rod load and position are measured simultaneously.
U.S. Pat. No. 4,583,915 discloses a pump-off controller which calculates the area defined by the minimum load, two user defined positioning lines, and the load at the time of calculation. This area is compared to a user-defined limit and the pump shuts down when the area value is below the limit.
U.S. Pat. No. 4,487,061 describes a system which detects abrupt increases in rod string load during the downstroke signaling fluid pound. The pump is shut down when fluid pound is detected.
U.S. Pat. No. 5,237,863 describes a method of preventing pump-off wherein maximum and minimum values are measured for both the rod string load and the rod position. The measured analog values are converted into digital values and expressed in terms of percentages. This prevents the pump from being shut down prematurely due to a high liquid level in the well. At high fluid levels, the energy required to operate the pump is much reduced leading to a reduced area as calculated according to the above discussed methods. As percentages are used, the change in the area is automatically compensated and false shutdowns are avoided.
None of the prior art devices deal with rotary downhole pumps nor do they provide for a method to measure and control the fluid level in the well.